Patent document DE102009002485 discusses a method of how thin MEMS chips may be produced. The document discusses how particularly thin caps may be applied onto an existing MEMS substrate.
Patent document DE000010350036B4 discusses a method that makes it possible to produce very thin chips without back-thinning. For this purpose, buried cavities are created in a substrate. Semiconductor component are produced on the substrate. Trenches extending to the buried cavities are produced by separation etching, and the now very thin semiconductor component are thereby separated. Usually, cavities are arranged in such a way that following the creation of the separation trenches the chips are still held by a small web. If the chips are picked off the substrate, this web is broken mechanically. The creation of the buried cavities is very painstaking and expensive. The cavities also cause limitations in process control with respect to stability and thermal coupling. Moreover, this method is not suitable for MEMS sensors having sensitive free-standing MEMS structures (acceleration and rotation-rate sensors) since vibrations are produced in the MEMS component when tearing off the small webs, which can destroy the movable structures of the MEMS component.
As an alternative for producing thin semiconductor components, methods were further developed in which essentially the substrate is ground down from behind until only very thin wafers remain. Such methods are of course also very painstaking, but have the essential advantage that the semiconductor substrate and the semiconductor production process do not have to be changed.
Such a process is not suitable for producing thin MEMS components such as acceleration sensors or rotation-rate sensors. When grinding the substrate, vibrations are coupled into the substrate, which cause the freely movable structures of the MEMS sensors to break. Furthermore, a great pressure is exerted on the substrate when grinding the substrate and removing material. The MEMS components have a cavity in which the exposed MEMS structures are situated. This cavity would be squeezed together by the applied pressure, and the MEMS structures would be thereby destroyed. Even if the thickness is chosen to be so thick that the cavity would not be squeezed together completely in the grinding process, the cavity would be bent inward and the grinding process would not be able to produce an even surface.
Processes without mechanical removal such as wet etching or plasma etching cannot be considered as alternatives since they have a highly inhomogeneous removal rate across the wafer and are therefore not suitable for producing very think chips having a precisely defined thickness.